The object of this invention is to provide a method for improving the accuracy of measurements, distance measurements in particular, made when using a laser interferometer.
When precise measuring accuracy is required, for example in the engineering industry, laser interferometers are used in distance measuring because they are more accurate than other methods. However, the accuracy of the laser interferometer depends on various factors, such as the dependency of the laser beam wavelength on the refractive index of the medium. Since distance measurements made using laser interferometers are based on the wavelength of light, it is essential to know the refractive index of the medium to obtain sufficiently accurate results. In practice, the medium in these measurements is air whose refractive index is a function of air pressure, temperature, humidity, and concentrations of gases such as carbon dioxide. Air temperature changes significantly due to the fast and constant movements of air masses caused by temperature differences between the various points in the measurement region and due to aerodynamic effects. Because of changes in air temperature, the integral transient temperature along the laser beam path of the laser interferometer cannot be accurately measured with the currently known thermometers used in laser interferometers.
In commercially available laser interferometer equipment, the refractive index of the air is determined by measuring the above quantities in air and then calculating their effects on the wavelength. For this purpose, the currently available equipment incorporate sensors to measure air temperature, pressure, and sometimes also humidity. Typically, there are one to three thermometers for air temperature measurement.
The air thermometers currently used in laser interferometers are based on the phenomenon that the sensor temperature becomes the same as the ambient air temperature. These thermometers only allow measuring of temperatures at single points adjacent to the laser beam path. These thermometers cannot measure temperatures at each point along the whole laser beam path, as is actually required. Another disadvantage is that, due to delays in the response times in the currently known temperature sensors of laser interferometers, the measurement results always lag behind the real instantaneous temperature at each measurement point. Consequently, these devices only allow measuring of integral temperature values at specific measurement points during a specific period of time; they do not allow measuring of constantly changing transient temperatures. Applying measurement methods which only allow measuring of temperature values at single points and with a long response time, and considering the resulting values as the integral instantaneous temperature along the whole laser beam path, results in an error which is the most significant among the error components affecting the accuracy of laser interferometer measurements.
The object of this invention is to provide a method and equipment which solve the problem above. In addition, this invention provides a method for correcting the laser interferometer measurement results, taking into account the transient changes in air temperature along the whole laser beam path.
The object of this invention can be achieved by using a method whose distinctive aspects are described in the claims.
The method according to this invention consists of determining the air temperature values along the whole laser beam path of the laser interferometer by measuring the speed of sound travelling the same path, and using the obtained value to calculate the correction depending on air temperature to the measured value, particularly to distance measurements. The expression xe2x80x98same pathxe2x80x99 means here that the soundwaves and laser light waves start from the same point or, in practice, from points near each other, and arrive at the same point or, in practice, at points near each other, and that these waves travel simultaneously. The correction to the laser interferometer measurement result, particularly to the distance measurement result, can be calculated from the air temperature value. This method gives an accurate picture of the transient air characteristics which affect the accuracy of the laser interferometer.
In an embodiment according to this invention, the speed of sound is measured using the value of length or distance measured simultaneously using a laser interferometer. When the method according to this invention is used, the accuracy of the measurements made using a laser interferometer improves considerably. The described method can be applied to commercially available laser interferometers without alterations or modifications and the equipment according to this invention can also be installed to them. In addition, the method and equipment are simple and inexpensive to implement, also the equipment is inexpensive to produce and to use.
This invention is based on the phenomenon that the refractive index of the medium (in practice, air) known from the wave theory of light, which affects the wavelength of the laser beam, and the speed of sound in air, known from the theory of acoustics, both depend on the same factors, namely air pressure, temperature, humidity, and concentration of carbon dioxide, and are thus comparable to each other in a known manner. A further advantage is that the soundwaves do not interfere with the laser light beam transmitted by the laser interferometer and it is possible for the soundwaves and the laser beam to travel the same path simultaneously. Thus, transient changes in air characteristics have the same effect on the soundwaves and the given laser beam. Since the relative effect of a change in air temperature, is over 1000 times stronger on the speed of sound in air than on the refractive index of air, it is possible to achieve very precise measurements.
The method according to this invention proposes that the determination of the correction to the length measurement result, which depends on the air temperature along the path of the laser beam and which is obtained with the laser interferometer, is based on the fact that when sound wave travels the same path as the laser beam through air, instantaneous air temperature at each point along the path affects the speed of sound wave. The time that the soundwave takes to travel through air is inversely proportional to the integral instantaneous temperature along the soundwave path. Besides air temperature, the speed of sound in air depends on other factors, the effect of which is tens or hundreds of times smaller than the effect of air temperature. These factors have a relatively uniform effect on the air conditions in the whole region, unlike air temperature which changes significantly from point to point and varies with time. These factors include air humidity, pressure and concentrations of different gases such as carbon dioxide.
A further advantage provided by the invention measurement method is that the response time is zero at each point of the laser beam path and measurement of the whole laser beam path only lasts the time that the soundwave takes to travel the path. This is the essential difference and overwhelming advantage in the principle of the proposed method compared to the slower air thermometers currently used in laser interferometers.
In a preferable embodiment of this invention, the sound transmitters and receivers are located in the laser interferometer or in its proximity so that the path and axis of the laser beam and soundwaves are as close to each other as possible and the soundwaves travel in parallel and symmetrically with respect to the laser beam. In this manner the measurement errors are reduced.